1. Technical Field
The present invention relates to wireless data communication and, more particularly, to improving packet-data communication for mobile stations via wireless wide area networks.
2. Description of Related Art
More people than ever are using mobile stations, such as cell phones and personal digital assistants (PDAs), to connect to wireless wide area networks (WWANs), which are also referred to as wireless communication systems, cellular communication systems, wireless networks, radio access networks, wireless access networks, and by other names. Service providers typically operate these WWANs to provide both voice and data services using a wireless communication format such as Code Division Multiple Access (CDMA), or another format.
Mobile stations (such as cellular telephones) typically store a “preferred roaming list” (PRL), which includes a prioritized listing of communication systems (such as WWANs) that the mobile station may access. In a CDMA system for instance, a PRL typically includes (i) an “acquisition table” that instructs the mobile station as to which radio frequency (RF) channels to search and (ii) a “system table” that specifies, for each allowed system, a system identification code (SID) and a network identification code (NID). Each CDMA base station broadcasts its SID and NID in system overhead messages. Thus, when a mobile station detects an allowed SID-and-NID combination, the mobile station may connect to the identified system.
In practice, an initial or default PRL is typically loaded into a mobile station before a wireless carrier distributes the mobile station to a subscriber. The carrier may thereafter load a new PRL into the mobile station using a physical connection (perhaps at a customer service center) or perhaps by using over-the-air service provisioning (OTASP), relevant aspects of which are described in “Over-the-Air Service Provisioning of Mobile Stations in Spread Spectrum Systems,” EIA/TIA IS-683-C (Mar. 20, 2003), which is incorporated herein by reference. In operation, mobile stations connect with various WWANs listed on their PRLs according to the listed priorities, and perhaps according to other logic.
As mentioned above, mobile stations connect to WWANs to engage in both voice and packet-data communications. The packet-data communications may take the form of, as examples, web-browsing communications, e-mail communications, Voice over Internet Protocol (VoIP) communications, and/or any other type of packet-data communications. To engage in these packet-data communications, mobile stations may use a packet-switched protocol such as the Internet Protocol (IP), relevant aspects of which are described in “Internet Protocol,” RFC 791 (September 1981), which is incorporated herein by reference.
Furthermore, to handle their own mobility, some mobile stations may use a mobility protocol known as “Mobile IP,” relevant aspects of which are described in “IP Mobility Support for IPv4,” RFC 3344 (August 2002), which is incorporated herein by reference. Mobile IP is essentially a way of constructing and routing IP packets to allow devices to change their point of attachment to the Internet while maintaining an at least semi-permanent IP address, known as a “home address,” to which other nodes may address packets.
In Mobile IP, mobile stations have a particular network known as a “home network.” When the mobile station is attached to its home network, it may receive packet data addressed to its home address on that network. When the mobile station is attached to another network, known as a “foreign network,” the mobile station acquires a “care-of address.” This address may be either an address assigned to the mobile station on the foreign network (“co-located care-of address”), or an address of a device on the foreign network known as a “foreign agent,” through which the mobile station conducts communications (“foreign-agent care-of address”).
Either way, the mobile station registers the care-of address with a device on its home network known as a “home agent.” When other nodes on the Internet address packets to the mobile station's home address, those packets will be routed to the mobile station's home network. There, the home agent intercepts the packets and uses encapsulation to tunnel the packets to the mobile station's registered care-of address. In the case of a co-located care-of address, the packets will arrive at the mobile station, which will then decapsulate the packets. In the case of a foreign-agent care-of address, the packets will arrive at the foreign agent, which will decapsulate the packets and then forward them to the mobile station.
Thus, packets sent to the mobile station from other nodes on the Internet are routed to the mobile station's home agent, and then to the mobile station. Packets sent from the mobile station to other nodes may or may not traverse the mobile station's home agent. If “triangular routing” is used, the mobile station addresses the outgoing packets directly to those other nodes. If “reverse tunneling” is used, the mobile station encapsulates packets that are addressed to the other nodes inside packets addressed to the home agent. Those packets are received by the home agent, decapsulated, and then forwarded to the other nodes.
In general, the greater the distance and number of nodes a packet must traverse, the more time it will take the packet to arrive at its destination. Conversely, the smaller the distance and number of nodes a packet must traverse, the less time it will take the packet to arrive at its destination. Thus, in the context of Mobile IP, the choice of a mobile station's home agent could impact the amount of time that packets sent to (and perhaps by) the mobile node take to reach their destination. Furthermore, this choice may also impact the amount of traffic a given network or group of networks is tasked with handling: the longer the packets take to reach their destination, the larger the number of packets that will be traveling at any given time.